The invention relates to a motor and in particular to a motor for a fan.
Motors for driving fans via magnetic torque generated therebetween are the currently prevailing model. The motor comprises a rotor and a stator. Either the rotor or the stator is a permanent magnet; the other is an electromagnet. The electromagnet is formed by applying power on windings to generate magnetic field, repelling the magnetic field of the permanent magnet. The repellant torque therebetween, however, must be as large as possible to drive the rotor. A popular solution is to enhance the output power of the motor, by increasing the amount or the outer diameter of the silicon steel plate or increasing the winding wound on the silicon steel plate for example.
FIG. 1A shows a conventional four-pole motor. To ensure that a rotor 12 can be activated without causing a dead angle situation or inability start, an asymmetric arc or r-angle 111 is formed on a magnetic pole of a silicon steel plate of a stator 11, or a stator 11 is designed with reluctance unbalanced. A winding 13 is wound around the stacked silicon steel plate. The rotor 12 is a symmetrical ring structure with equal magnetization and further comprises a metal shell 121 and a permanent magnet 122. A magnetizing method for the permanent magnet 122 of the rotor 12 is shown in FIG. 1B, set as N-S-N-S. A distance of gap 14 between the stator 11 and the rotor 12 is changed or non-uniform (as shown in FIG. 1A) to generate reluctance torque forcing the rotor 12 to shift its balanced position, such that the rotor 12 can start smoothly, and thus drive the motor.
However, the mentioned motor alleviates the start difficulty problem but increases torque ripple during operation and further produces vibration and noise problems. Further, the stator includes a plurality of silicon steel plates with the same shape. Some of which are modified to form an asymmetrical arc or r-angle thereon, thereby increasing the process difficulty and cost, and the torque ripple of the motor is difficult to eliminate. Further, the gap between the stator and rotor is large, causing greater loss of the magnetic circuit and magnetic flux.